Generally, in preparing liquid crystal display devices, liquid crystal cells comprising liquid crystals and polarizing plates are basically required and suitable adhesive layers or pressure-sensitive adhesive layers have to be used for binding them. In addition, for improving functions of liquid crystal display devices, a phase retardation plate, a compensation plate for wide view angle, a brightness enhancement film, and the like may be used, with additionally adhered to the polarizing plate.
Major structure forming a liquid crystal display device comprises, generally, a uniformly aligned liquid crystal layer; a polarizing plate with a multi-layer structure, incorporated into a pressure-sensitive adhesive layer or an adhesive layer, based on a liquid crystal cell consisted of a transparent glass plate or plastic sheet material containing a transparent electrode layer; a phase retardation plate; and an additional functional film layer and the like.
The structure of polarizing plate is one comprising an iodine compound or a dichroic polarizing material aligned in a certain direction. To protect these polarizing elements, multi-layers are formed on both sides using a protective film such as triacetyl cellulose (TAC). In addition, the polarizing plate may additionally comprise a phase retardation film, or a compensation film for wide view angle such as a liquid crystal type film, in a shape having a unidirectional molecular alignment.
The aforementioned films are made of materials having different molecular structures and compositions, and so have different physical properties. Especially, under a high temperature or a high temperature and humidity condition, the dimensional stability according to shrinkage or expansion of materials having a unidirectional molecular alignment is insufficient. As a result, if the polarizing plate is fixed by a pressure-sensitive adhesive, deforming stress by shrinkage or expansion of polarizing plates under a high temperature or a high temperature and humidity condition is in remained state, and thereby light leakage occurs in the part on which stress is focused.
In a method for improving the above light leakage phenomenon, it is needed to reduce the shrinkage of the polarizing plates in a high temperature or a high temperature and humidity condition. However, it is very difficult to remove the stress generated from a liquid crystal panel to which a polarizing plate consisted of materials having different physical properties is attached. Another method for improving the light leakage phenomenon is one designed to provide a stress release function to a pressure-sensitive adhesive layer that fixes a polarizing plate to a liquid crystal panel. Generally, a pressure-sensitive adhesive includes rubbers, acrylics, and silicones and the like. Among these, acrylic pressure-sensitive adhesives are advantageous in terms of adhesion property, optical property, durability, and weatherability, and so have been most widely used for manufacturing pressure-sensitive adhesive composition for polarizing plates.
In order to add said stress release function to the pressure-sensitive adhesive layer, usual pressure-sensitive adhesives are designed so that they may have a large magnitude of creep against an external stress and be easily deformed. As a representative method has been attempted that an acrylic high molecular weight polymer comprising a reactive cross-linking functional group reactive with a multi-functional cross-linking agent is reacted to set an appropriate cross-linking density, satisfy durability under a high temperature or a high temperature and humidity condition and improve the stress release of pressure-sensitive adhesives. It is known to use isocyanates and epoxys multi-functional cross-linking agents as a preferred method for forming a cross-linking density of acrylic pressure-sensitive adhesive. Such a method for preparing pressure-sensitive adhesives may control physical properties of final pressure-sensitive adhesives by mixing a cross-linking agent and an acrylic polymer containing a functional group reactive therewith, coating the mixture on a substrate, heating and drying, followed by a reaction of the acrylic polymer with the cross-linking agent. However, when pressure-sensitive adhesives are prepared by such a method, it takes very long aging time. In addition, since cross-linking does not occur yet immediate after drying, the pressure-sensitive adhesive has very low modulus and thereby may greatly cause problems for workability such as a trouble that must be stored in low-temperature storage, or To solve the above problems by increasing the cross-linking speed of pressure-sensitive adhesive, it is a preferable method for cross-linking it by light, instead of a chemical cross-linking reaction. For example, JP Unexamined Patent Publication No. S49-005145 discloses an attempt that an acrylic polymer having a double-bond in a side chain is prepared and cross-linked with radiation. In addition, JP Unexamined Patent Publication No. 2001-107005 discloses a method comprising mixing 100 parts by weight of a high molecular weight acrylic copolymer having a weight average molecular weight of 500,000 to 2,000,000 and 1 to 100 parts by weight of an acrylic polymer having a weight average molecular weight of 200,000 to 1,000,000 and a double bond in a side chain, and then injecting a photo-initiator to the mixture, followed by radiation cross-linking. In addition, since a high molecular weight acrylic polymer is used, in which a number of double bonds are introduced in the side chain, it is difficult to regulate an appropriate cross-linking structure. Since double bonds in the side chain remain, it is more likely to affect optical properties.
On the one hand, in U.S. Pat. No. 4,144,157, an aspect attempting the radiation cross-linking is disclosed, using an acrylic copolymer in which a photo-initiator is introduced in the side chain by a photo-initiator copolymerizable with a acrylic monomer. In JP Unexamined Patent Publication No. 1995-032797, there is an aspect that a pressure-sensitive adhesive for skin is prepared via preparing an acrylic polymer using an aromatic ketone photo-initiator monomer copolymerizable with an acrylic monomer, followed by radiation cross-linking thereof. In U.S. Pat. No. 6,586,491, an aspect using in a pressure-sensitive adhesive for hot melt is published, wherein an acrylic polymer having a molecular weight of more than 500,000 or more and an acrylic copolymer having a molecular weight of 500,000 or less and introducing a photo-initiator in the side chain are mixed. However, the above arts do not disclose technical ideas about regulating cross-linking structures, thereby regulating stress release properties and durability of pressure-sensitive adhesives.